1. Field of the Invention
This invention relates to a semiconductor-laser-pumped solid state laser in which a solid state laser medium is pumped by a semiconductor laser (laser diode), and more particularly to such a semiconductor-laser-pumped solid state laser in which heat dissipation from the solid state laser medium is enhanced.
2. Description of the Related Art
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-189783, there has been known a semiconductor-laser-pumped solid state laser in which a solid state laser medium doped with a rare earth element such as neodymium is pumped by a laser beam produced by a semiconductor laser.
Recently, in such semiconductor-laser-pumped solid state lasers, there have been increasing demands toward miniaturization, increasing the output power and improving the beam quality (oscillation in single transverse mode). To meet these demands, use of a solid state laser medium higher in pumping beam absorption efficiency is required and the pumping laser beam is required to be higher in power and smaller in beam diameter.
Thus, use of Nd:YVO4, Er:YAG, and the like which are high in pumping beam absorption efficiency has been considered. Further, it has become possible to expose the solid state laser medium to a pumping laser beam which is high in output power and small in beam diameter by virtue of increase in output per unit area of the semiconductor laser and improvement in light condensing technique, for instance, in a fiber coupling.
However attempts to meet the requirements on the solid state laser medium and the pumping laser beam often greatly increase the temperature of the solid state laser medium and give rise to a problem such as thermal breakdown, thermal lens or the like.
As a structure for promoting heat dissipation from the solid state laser medium, there has been known a structure disclosed in Japanese Unexamined Patent Publication No. 8(1996)-88428 in which the front end face of the solid state laser medium (the end face opposite to the pumping light incident side end face, i.e., the end face from which the pumping laser enters the solid state laser medium) is mounted on a holder which is formed of a material high in thermal conductivity such as copper and the holder is in contact with a cooling element by way of a heat transfer medium.
Further there have been proposed structures in which the front end face of the solid state laser medium is kept in a close contact with a cooling element by way of foil or a side face of the solid state laser medium is kept in contact with a heat sink, as disclosed for instance in Japanese Unexamined Patent Publication No. 8(1996)-213689.
However even with these structures, heat dissipation from the solid state laser medium is not sufficient and the problem of thermal lens and/or thermal breakdown cannot be avoided.
In view of the foregoing observations and description, the primary object of the present invention is to provide a semiconductor-laser-pumped solid state laser in which heat can be sufficiently dissipated from the solid state laser medium.
In accordance with a present invention, there is provided a semiconductor-laser-pumped solid state laser in which a solid state laser medium doped with a rare earth element such as neodymium is pumped by a laser beam produced by a semiconductor laser, wherein the improvement comprises that a pumping light incident side end face of the solid state laser medium is fixed to a holder.
It is preferred that the holder be provided with a through hole through which the pumping laser beam impinges upon the pumping light incident side end face of the solid state laser medium and the inside dimensions of the through hole change continuously or stepwise in the direction of thickness of the holder.
More specifically, it is preferred that the through hole is circular in cross-section and tapered toward the solid state laser medium to conform to the shape of the pumping laser beam which is substantially circular in cross-section and converges toward the solid state laser medium.
It is further preferred that the end face of the solid state laser medium opposite to the pumping light incident end face is in contact with a heat dissipation plate. Preferably the heat dissipation plate is directly in contact with a cooling element or in contact with a cooling element by way of a member which is high in heat conductivity.
Resonator mirrors and a polarization control element may also be fixed to the holder for holding the solid state laser medium and the temperature of the holder may be controlled by a cooling element so that the length of the resonator is kept constant. In this case, the cooling element may double as the cooling element which is in contact with the aforesaid heat dissipation plate.
It is preferred that the holder be of metal which is high in heat conductivity.
The arrangement of the present invention is more effective when Nd:YVO4 or Er:YAG which are high in pumping beam absorption efficiency is used as the solid state laser medium.
We have found that the problem of thermal lens and/or thermal breakdown, which is apt to occur when a solid state laser medium high in pumping beam absorption efficiency is used or a high output small diameter pumping laser beam is used, is due to the following point.
These will be described hereinbelow. FIG. 6A shows a case where a pumping laser beam 2 which is relatively low in output and relatively large in divergent angle (large in beam diameter) enters a solid state laser medium 1 which is relatively low in pumping beam absorption efficiency.
FIG. 6B shows a case where a pumping laser beam 2xe2x80x2 which is relatively high in output and relatively small in divergent angle (small in beam diameter) enters a solid state laser medium 1xe2x80x2 which is relatively high in pumping beam absorption efficiency.
In FIGS. 6A and 6B, the hatched portions show main heat generating regions. In the former case (shown in FIG. 6A), heat is generated from a relatively wide area. To the contrast, in the latter case (shown in FIG. 6B), heat is generated from a limited area near the pumping laser beam incident end face and accordingly unless the generated heat is efficiently dissipated, the temperature of the solid state laser medium is greatly increased to cause the problem of thermal lens or thermal breakdown.
In the semiconductor-laser-pumped solid state laser in accordance with the present invention, since the pumping laser beam incident end face of the solid state laser medium is fixed to the holder, the generated heat can be well dissipated through the end face and the holder, whereby the temperature of the area near the pumping laser beam incident end face is prevented from being excessively elevated and the problem of thermal lens and/or thermal breakdown can be avoided.
When the holder is formed of metal which is high in heat conductivity, the generated heat can be better dissipated through the end face and the holder.
When the holder is provided with a through hole through which the pumping laser beam impinges upon the pumping light incident side end face of the solid state laser medium and the inside dimensions of the through hole change continuously or stepwise in the direction of thickness of the holder, the surface area of the wall portion circumscribing the through hole becomes larger than when the inside dimensions of the through hole are uniform in the direction of thickness of the holder, and heat dissipating effect is enhanced.
Further when the through hole is circular in cross-section and tapered toward the solid state laser medium to conform to the shape of the pumping laser beam which is substantially in cross-section and converges toward the solid state laser medium, the contact area between the solid state laser medium and the holder is wider and heat dissipating effect is further enhanced.
Further when the end face of the solid state laser medium opposite to the pumping light incident end face is in contact with a heat dissipation plate, heat dissipating effect is further enhanced.
When the heat dissipation plate is directly in contact with a cooling element or in contact with a cooling element by way of a member which is high in heat conductivity, heat dissipating effect is further more enhanced.